When finely divided coal is burned in suspension in a boiler, as in a coal-fired electric power generating plant, combustion converts ash and magnetite components of the coal into "flyash" particles. These depart the combustion chamber suspended in the gaseous and vaporous products of combustion. This suspension is then passed through an appropriate device, such as an electrostatic precipitator, to separate the particulate matter from the gases and vapors, thereby reducing the concentration of solid pollutants discharged into the atmosphere through the plant smoke-stack.
However, "flyash" is not a precise term of definition. For instance, the materials described as "flyash" can vary widely from plant to plant, depending upon a variety of factors including composition of the coal, furnace and separator design and combustion conditions. Moreover, the "flyash" being collected at a given plant at a given instant includes particles of widely varying particle size, density, shape, porosity, internal structure, surface chemistry and other properties. Accordingly, prior literature references setting forth specific particle sizes, densities, chemical analyses and the like for a given flyash generally constitute average or approximate values for a multitude of particles in a sample, in which many of the particles differ widely from the average values. Thus, "flyash" may be aptly described as a composition of diverse particled, which varies from plant to plant. Hereinafter, the unmodified term "flyash" refers to this varying, diverse material.
The abundance and low cost of flyash has created interest in finding uses for it. Early attempts at utilization involved the use of flyash itself. Thereafter, more and more effort focused on isolating specific fractions of the flyash and preparing specially modified forms thereof, such fractions or modifications being chosen based upon what was thought most advantageous for the contemplated end use.
For example, U.S. Pat. No. 2,892,240 discloses the use of flyash as a filter aid in filtration of a city water supply. The flyash is a modified flyash prepared by moistening flyash with a volatile solvent for metal salts present in the flyash, drying the moistened material and subsequently calcining the dried material.
U.S. Pat. No. 2,987,406 discloses the preparation of cement containing a high density flyash fraction. The latter is composed of sphere-like particles consisting essentially of a silica-alumina glass envelope and matrix having dispersed therein crystallites of a ferromagnetic spinel, said particles containing between about 70 and about 95% of iron, expressed as Fe.sub.2 O.sub.3.
U.S. Pat. No. 2,987,408 discloses Portland Cement containing a flyash fraction composed of discrete, substantially spherelike particles having a particle size less than about 200 mesh and a specific gravity from about 2.1 to about 2.6, consisting essentially of glass comprising silica and alumina, and preferably having a surface film or coating comprising a water soluble salt, which film enhances the pozzolanic action of the particles.
U.S. Pat. No. 3,533,819 suggests dividing flyash into a sintered aggregate product, an iron concentrate product, an improved pozzolan product and a carbon product by (1) separating flyash into a first iron concentrate product and a low iron content fraction; (2) separating the low iron content fraction into a second, improved, pozzolan product and another fraction; (3) screening the latter fraction to obtain a third, carbon product containing a minimum of 25% by weight of carbon and a sinter fraction; and (4) agglomerating the sinter fraction while adjusting the amounts of the first, second and third products removed from the production stream to maintain substantially uniform quality in the fourth product.
Depending on raw material and plant variables discussed above, some power plants product flyash having a relatively high percentage by volume, e.g. up to 30%, of hollow spherical particles; other plants do not. U.S. Pat. No. 3,585,155 discloses flyash-asphalt mixtures containing the type of flyash having hollow spheres, which in this instance ranged in average particle size from 1 to about 80 microns.
It has been proposed in U.S. Pat. No. 3,765,920, to prepare bloated aggregates by mixing "flyash" with sodium or ammonium hydroxide and a water soluble carbohydrate, forming the mixture into aggregates, drying them and firing them at a sufficiently high temperature to cause fusion and bloating of the aggregates. It was also suggested that such aggregates be formed into a variety of shaped products such as acoustical or heat insulating blocks or panels with or without the assistance of a binder.
U.S. Pat. No. 3,901,991 discloses production of non-combustible shaped articles containing a silica material, lime, mineral fibers, water and a soluble inorganic binder, in which the silica filler is flyash having a particle size of approximately 5 microns.
Heretofore, flyash and flyash components have been employed as fillers and extenders for filled synthetic polymer composites. As evidenced by U.S. Pat. No. 2,895,935, and others, much of the activity in this area of utilization apparently involved incorporation of flyash per se in thermoset and thermoplastic resin systems. U.S. Pat. No. 3,991,005 discloses composites of thermosetting or thermoplastic resin and flyash in which the individual flyash particles are characterized by non-porous structure and irregular, rough shapes having a heterogenous size distribution less than 50 microns, e.g. 35-45 microns, which are said to give composites with good structural properties. By way of contrast, U.S. Pat. No. 3,755,242 teaches that flyash with a "sub-micron" particle size yields low density (4 pounds per cubic foot) foams which are friable, whereas foams of about the same density prepared with titanium dioxide and concrete dust are strong. Several applications of flyash as filler for polyesters and polyester foams are disclosed in U.S. Pat. Nos. 3,805,533, 3,884,844 and 3,896,060. U.S. Pat. No. 3,577,380 teaches that flyash particles include hydroxyl groups on their surfaces which can react with polyfunctional isocyanate compounds and elastomers having pendant carboxyl groups, so that the flyash contributes to stiffening of the resulting polymer.
Notwithstanding the advantages of flyash suggested in the foregoing patents, there has been an apparent need for further improvements. This need has manifested itself in growing enthusiasm for a particular flyash fraction known as cenospheres.
In this connection, U.S. Pat. No. 3,830,776 proposes the preparation of shaped articles comprising a polymeric matrix in which is distributed beads having a specific gravity of less than 1.25, separated from flyash by selective floatation and having a particle size range of about 50 to 200 microns. So that they will have adequate crush strength, the beads are subjected to hydrostatic pressure in a high pressure cell and cycled several times from a 50 psig threshold pressure to a test pressure and back, while monitoring crushing acoustically, and are checked for extent of crushing by the change in volume of the water in the test cell and by the density of the beads before and after the pressure sequence.
In a paper entitled "Cenospheres", paper no. A-4, 3rd International Ash Utilization Symposium, 1973, Dr. J. W. Pedlow reviews the properties, uses and availability of "floaters", hollow spheres 20-200 microns in diameter having a particle density in the range of 0.4 to 0.6 grams per cubic centimeter, which decrepitate on heating to 300.degree. C. Dr. Pedlow's company has used floater cenospheres as a lightweight filler to reinforce fire retardant and burn-through resistant vinyl mastic employed as an adhesive for electrical cable tape. However, Dr. Pedlow also reports on the work of other researches. The replacement of glass microspheres with cenospheres in syntactic foam was reported. Use of cenospheres in liquid epoxy systems for electronic castings was also disclosed. Incorporation of cenospheres in rigid foamed-in-place urethane resins was said to lower the cost and improve the physical properties of the resultant foam. It was also stated that polyester-cenosphere composites had been molded into ornamental plants. Unfortunately, as Dr. Pedlow has also reported, floater cenospheres have a limited availability in that they are produced by some power plants and not others. An investigation disclosed that three plants in the Kanawa Valley supply most of the cenospheres which are being used, while seventeen power stations in the Ohio River Valley Basin had virtually no cenospheres in the flyash they produced.
H. Jan de Zeeuw and Roland V. Abresch have suggested that no-floating cenospheres may be recovered from beneath the surfaces of ash ponds; Ash Utilization, Proceedings: 4th International Ash Utilization Symposium, 1976, pages 386-395. This particulate material includes particles ranging in size from one third to two hundred microns and has a density of approximately 50 pounds per cubic foot (0.8 grams per cubic centimeter), with walls which are generally thicker than those of the floater cenospheres. It is indicated that the wall thickness of at least some of the particles can be up to thirty percent of the diameter of the sphere.
U.S. Pat. No. 3,917,547 discusses the use of cenospheres in polyurethane foams. It was suggested that the inclusion of up to 75% by volume of cenospheres improves the tear strength, compressive strength, and other properties of the foams. Since the results obtained are most directly related to the percentage of cenospheres present in the flyash, it is suggested to use flyash enriched in cenospheres recovered by dumping flyash into water and collecting the floating particles.
Apparently, those researchers who have given detailed consideration to synthetic resin/flyash interrelationships have attributed superiority to flyash or flyash fractions characterized by abundant quantities of hollow particles. Contrary to the expectations produced by these teachings, we have discovered a material which has excellent utility in synthetic resin composites, and which does not involve a requirement for an abundant content of hollow particles.